Multilevel selection 1, multilevel selection 2, and the Price equation: a reappraisal

The distinction between multilevel selection 1 (MLS1) and multilevel selection 2 (MLS2) is classically regarded as a distinction between two multilevel selection processes involving two different kinds of higher-level fitness. It has been invoked to explain evolutionary transitions in individuality as a shift from an MLS1 to an MLS2 process. In this paper, I argue against the view that the distinction involves two different kinds of processes. I show, starting from the MLS2 version of the Price equation, that it contains the MLS1 version if, following the assumption that a collective constitutively depends (i.e., mereologically supervenes) on its particles, one considers that a necessary map between fitness at two levels exists. I defend the necessity of such a map, making the distinction between MLS1 and MLS2 a matter of perspective and limited knowledge (i.e., epistemic limitations) rather than objective facts. I then provide some reasons why the MLS1/MLS2 distinction nonetheless has some pragmatic value and might be invoked usefully in some contexts, particularly within the context of explaining evolutionary transitions in individuality.